1. Field of the Invention
The invention relates to a magnetic resonance (MR) method utilizing both the Magnetization Transfer Contrast (MTC) effect and fat (or lipid) suppression.
2. Description of the Prior Art
A method of this kind is known from B. Pierce et al, "Three-dimensional Gadolinium-enhanced MR Imaging of the Breast: Pulse Sequence With Fat Suppression and Magnetization Transfer Contrast", Radiology 1991, No. 181, pp. 757-763. This method employs two RF pulses sequentially, the first, applied 2 KHz off the Larmor frequency of water protons to induce the MTC effect and the second applied at the Larmor frequency of fat bound protons and having a net zero flip angle for fat, but exciting transverse magnetization in water.
Magnetization Transfer Contrast offers the opportunity to produce new contrasts which may prove valuable in diagnosis or identification of tissue exhibiting pathology. It derives from magnetization exchange between proton spins in restricted motion .sup.1 H.sub.r, which are not directly observed in the usual MRI experiments on account of an extremely short spin-spin relaxation time T.sub.2, and the normally observed "free" water proton spins .sup.1 H.sub.f. The short T.sub.2 of the restricted motion protons .sup.1 H.sub.r implies a broad linewidth of the restricted motion proton pool. Destruction of the longitudinal magnetization of the .sup.1 H.sub.r immobile protons, i.e. saturation, by RF irradiation within (or near) the linewidth of the immobile protons and outside the linewidth of the free water protons, causes a decrease in the longitudinal magnetization of those free water protons .sup.1 H.sub.fc which are coupled to the immobile protons by the magnetization exchange pathway. The longitudinal magnetization of free water protons .sup.1 H.sub.fn which are not coupled to immobile protons is not influenced.
The resultant contrast between free water protons dependent upon their coupling to immobile protons was quantified in S. Wolff et al, "Magnetization Transfer Contrast (MTC) and Tissue Water Proton Relaxation in Vivo", Magn Res Med 1989, No. 10, pp. 135-144. Wolff et al assumed that irradiation at a frequency which was offset at least 5 kHZ and no more than 50 KHz from the Larmor frequency of water protons would be selective to the MTC effect. B. Hu et al (Book of Abstracts, Society of Magnetic Resonance in Medicine, 9th Annual Meeting, 1990, p. 352) reported utilizing a short intense 1-2-1 binomial pulse yielding 0.degree. net flip angle for free water protons while saturating the immobile water protons. Also mentioned was that H.sub.r magnetization could be eliminated by a single pulse with a constant amplitude of 2500 radians/sec (approximately 400 Hz) for maximal rate of decay and played for 1 to 2 ms.
The use of a binomial pulse for fat suppression is also known for producing no net flip angle for water protons but the center-to-center spacing between spaced apart sub-pulses is chosen so that the binomial pulse is frequency selective to the Larmor frequency of fat and has a net flip angle at that frequency which s the sum of the flip angles of the sub-pulses. Fat-selective pulses of 90.degree. or 180.degree. net flip angle may be implemented depending upon the suppression scheme.